Acids and bases are important intermediates for a wide variety of products made by the chemical industry. After processing and use these find their way back to nature as salts. A logical route for completing the cycle would be to regenerate the acids and bases directly from these salts. Electrolysis of brine to generate chlorine and caustic soda, in a certain sense, is such a process. Another process is electrodialysis, using bipolar membranes to directly generate acids and bases from their salts. The process is electrically driven and the splitting of the salt to generate the acid and base occurs in an aqueous medium. The process is conceptually a simple one and can be represented by the equation: ##STR1##
To effect and maintain separation of the various species, ion exchange membranes are used. The most crucial of these is the bipolar membrane, so called because it is composed of two distinct parts which are selective to ions of opposite charges. Under the influence of an applied direct current, such a sandwich membrane is capable of forcibly dissociating water to form equivalent amounts of hydrogen and hydroxyl ions. Used in conjunction with other cation- and anion- selective (i.e., monopolar), membranes, the assembly constitutes a potentially economical water splitting apparatus that generates acid and base.
The standard free energy for a process that converts water to one molar hydrogen and hydroxyl ions at 25.degree. C. is 19,100 cal/mole. For a reversible process, i.e., a process approaching zero current density, this translates to an energy requirement of 0.022 kWh/mole at 25.degree. C. For production of caustic soda this is equivalent to an energy requirement of 500 kWh/ton. An efficient water splitting system is therefore capable of generating acid and base solutions at a fraction of the costs encountered commercially (2800-3500 kWh/ton).
Bipolar membranes can be prepared by many different methods. In U.S. Pat. Nos. 4,024,043 and 4,057,481 (both Dege et al) single film bipolar membranes are prepared from pre-swollen films containing a relatively high amount of an insoluble cross-linked aromatic polymer on which highly dissociable cationic exchange groups are chemically bonded to the aromatic nuclei to a desired depth of the film from one side only; subsequently, highly dissociable anionic exchange groups are chemically bonded to the unreacted aromatic nuclei on the other side of the film.
In Japanese Patent Publication Nos. 78-158638 and 79-7196 (both Tokuyama Soda Co. Ltd.), bipolar membranes are prepared by partially covering a membrane with a cover film, sulfonating the surface of the membrane not in contact with the cover film to introduce cation exchange groups, exfoliating the cover film, and introducing anion exchange groups on the exfoliated surfaces.
Bipolar membranes have also been prepared by bonding together separate anion and cation exchange films or membranes. The two monopolar membranes of opposite selectivity can be fused together with the application of heat and pressure. See, for example, U.S. Pat. No. 3,372,101 by Kollsman wherein separate cation and anion membranes are bonded together in a hydraulic press at 150.degree. C. at a pressure of 400 lb/sq. inch to form a two ply membrane structure.
However, bipolar membranes formed in this way suffer the disadvantage of high electrical resistance produced by the fusion. Furthermore these membranes are prone to bubble or blister and they are operable for only short time periods at relatively low current densities. These disadvantages make the bipolar membranes formed in this way unattractive for commercial electrodialysis operations.
In other relevant prior art, published by the present inventor in Electrochimica Acta, 31(9) 1175-1176 (1986), there is disclosed a method for the preparation of bipolar membranes whereby inorganic electrolyte solutions are brushed onto the faces of suitable anionic and cationic membranes, prior to the faces being pressed together. A variety of electrolyte solutions were found to be effective in facilitating the preparation of potassium hydroxide and hydrochloric acid from a potassium chloride solution. Unfortunately, it was found that membranes only remained effective for a few hours when 1 molar acid and base solutions were separated by a membrane and for a few weeks only when a membrane separated potassium chloride solutions.